The present invention relates generally to a power tool, and more particularly to a driver for nuts, bolts, screws and the like.
Particularly the construction industry utilizes more and more self-tapping screws which have a relatively small diameter and are formed with a pointed tip, or have a larger diameter and are formed with a cutting edge analogous to the edge found on drill bits. Such screws are used for making connections in soft material, such as wood, synthetic plastic or on sheet rock that is placed over a metallic base. The advantage of using such screws is that several previously necessary operating steps are combined in a single one, since the screw drills its own hole and in the same operating step is also threaded into the hole. The use of such screws is time saving and, therefore, labor saving and aids in cutting of construction expenses, especially where large projects are involved.
Special power tools have been developed for use with such screws, usually electrically powered power tools having a pistol grip and using a simple claw-type coupling having a forward claw ring which is connected with the tool holder or screw holder, and which when the tool is placed against the screw and the latter is pressed against the workpiece, is shifted axially against the force of a spring and brought into engagement with a second claw ring that is driven by the motor. Once the screw has been driven in, a slight release of the pressure exerted upon the tool in the direction towards the workpiece, is sufficient to bring the two claw rings out of engagement in order to thereby interrupt further transmission of motion to the shaft which turns the screw-engaging component of the tool.
Generally speaking, this type of equipment has been found to be highly advantageous in the construction industry and is widely used. However, the currently used self-tapping screws range in diameter from approximately 2.9 - 6.3 mm, that is they have a quite wide diameter range. For this wide range it is difficult to determine an optimum rotational speed for the tool, since the smaller-diameter type screws, which are very widely used, permit a high rotational speed which is desired for reasons of economy. Larger-diameter self-tapping screws, however, frequently require a higher torque than it is possible to deliver at the higher rotational speed.